The present disclosure relates to a multi-layer work function metal replacement gate, and in particular to layers of work function metals that conform to a shape of a trench structure and which are variable to adjust work function levels of a replacement gate structure.
Field-effect transistors (FETs) generate an electric field, by a gate structure, to control the conductivity of a channel between source and drain structures in a semiconductor substrate. The source and drain structures may be formed by doping the semiconductor substrate, and the gate may be formed on the semiconductor substrate between the source and drain regions. Alternatively, a source and drain structure may be formed on the substrate, and a channel may extend between the source and the drain on the semiconductor substrate. In such a structure, referred to as a finFET due to the fin-like shape of the channel, the gate structure may be formed on the channel.
The gate of a finFET, and in some non-finFETs, may be formed by a replacement gate process, or a process in which material, such as dummy material, is removed to form a trench, and the gate materials replace the removed material in the trench. In a finFET, the trench may be defined by a plurality of channels and the source and drain structures. In other FETs, as well as in finFETs, the trench may be formed by insulating separators, for example. The gate may be formed by depositing a work function metal in the trench and forming a metal gap fill on the work function metal. Titanium aluminum (TiAl) has been used as a replacement gate work function metal, but TiAl has been limited to non-conformal methods of application, such as physical vapor deposition (PVD), in which an upper surface of the deposited material does not conform to a shape of the surface on which the material is deposited, making TiAl less-than-ideal as a replacement gate work function metal. In addition, use of Al-based metal electrodes causes gate leakage current degradation due to a strong oxygen gettering effect.